Think of it this way. If enough people hadn't of voted for Obama, we would now have John McCain as president and Sarah Palin as vice president.
A vote for Obama and the Democrats was a vote for Lilly Ledbetter. It was a vote for a reasoned policy in the Middle East (if you doubt this, imagine what McCain would have done). It is a vote for an end to don't ask don't tell. All of these things would not have happened without the votes of reasonable people, many of who are disappointed with Obama on many fronts, hadn't voted.
Politics is a battleship, you make progress bit by bit, vote by vote and election by election. Just because Obama is not the magic genie of herculean virtue that we were expecting doesn't mean that his election wasn't a move (however disappointing) in the right direction.
Of course it is your decision, although if you don't vote you have no right to complain about what happens.
I sure wish I could use your vote if you aren't going to use it. I am trying to make things better and every bit helps.